system

We can rewrite the system of equations by replacing $x$ with $-2$ to get

$y-6=a$

$5y-10=2a$.

Multiplying the first equation by $2$, we have $2y-12=2a$.

Subtracting this from the second equation says that $3y+2=0$, so therefore $y=-2/3$, and so $a=-20/3$.